Mesoderm Formation in the Drosophila Embryo

Abstract

All muscle cells develop from the mesoderm, which is the middle germ layer in the early embryo. The mesoderm itself derives from the ventral cells of the blastoderm stage embryo. Therefore, the regulatory events controlling dorsal-ventral development in the oocyte and the early embryo are the earliest events in muscle formation. The first stage in dorsal-ventral development can be traced back to the oocyte, where Gurken-Torpedo signaling establishes dorsal-ventral asymmetry. The ventral half of the oocyte is then allowed to express Pipe, which serves to activate a series of serine proteases. These activation events ultimately lead to the stimulation of the Toll receptor in the ventral side of the early embryo. In the final stage of this Toll maternal cascade there is the formation of the Dorsal protein gradient in the ventral nuclei of the embryo blastoderm. The Dorsal gradient both activates and represses zygotic gene expression to establish mesodermal cell fate and promote mesoderm invagination. The invaginated mesoderm then differentiates into appropriate muscle tissue types according to further positional information.